A sintered rare-earth/iron/boron magnet, which is called “RFeB magnet” hereinafter, was introduced in 1982 and is steadily spreading their fields of commercial application as ideal materials for permanent magnets. They can be produced at low costs from neodymium, iron, boron and other materials abundantly present in nature. Moreover, their characteristics are much better than those of their predecessors. The major application areas of the RFeB magnets are: voice coil motors (VCMs) for actuating magnetic heads of hard disk drives (HDDs) used in computers; high-quality speakers; headphones; battery-assisted bicycles; golf carts; and magnetic resonance imaging (MRI) apparatuses using permanent magnets. They are also coming into practical use in drive motors for hybrid cars.
The RFeB magnet was discovered by the present inventors (see Patent Document 1) in 1982. Its main phase consists of a magnetically anisotropic, intermetallic compound of R2Fe14B having a tetragonal crystal structure. To obtain high magnetic characteristics, it is necessary to utilize a magnetic anisotropy. In addition to sintering, several methods have been proposed. For example, Japanese Patent No. 2561704 discloses a method including the steps of casting, hot working and aging treatment. Another method disclosed in U.S. Pat. No. 4,792,367 has the step of die upsetting of a quenched alloy. However, these methods are inferior to the sintering method with respect to both the magnetic characteristics and productivity. Sintering is the best method for obtaining a dense and uniform microstructure that is indispensable for permanent magnets.
[Manufacturing Process]
The process of manufacturing a sintered RFeB magnet includes the following steps: composition determination, dissolution, casting, pulverization, compression molding in a magnetic field, sintering, and heat treatment.
[Composition]
Since the discovery of the RFeB magnet, many techniques for improving the coercive force and other characteristics of the magnet have been invented, focusing on the effects of additional elements (e.g. Japanese Patent No. 1606420), heat treatments (e.g. Japanese Patent No. 1818977), and control of the crystal grain size (e.g. Japanese Patent No. 1662257). The most effective technique for enhancing the coercive force is the addition of heavy rare-earth elements (Dy and Tb) (Japanese Patent No. 1802487). Use of a large amount of heavy rare-earth elements assuredly improves the coercive force. However, it also lowers the saturation magnetization and accordingly decreases the maximum energy product. Furthermore, both Dy and Tb are rarely found in nature and also expensive, so that these elements cannot be used to produce motors for hybrid cars, which will gain more commercial demand in the future, or other industrial or domestic motors.
[Resolution]
Sintered magnets need to have a dense, uniform microstructure. In earlier years, they were typically manufactured by casting a molten alloy and pulverizing the cast alloy (e.g. Japanese Patent No. 1431617). Quenching the molten alloy by a strip-casting method suppresses the formation of alpha iron. This reduces the amount of nonmagnetic rare-earth elements and thereby increases the energy product (e.g. Japanese Patent No. 2665590 and Unexamined Japanese Patent Publication No. 2002-208509).
[Pulverization]
An RFeB alloy becomes easier to pulverize when it occludes hydrogen, because the hydrogen creates microcracks within the alloy (Japanese Patent No. 1675022). The most popular pulverization method is a jet-mill pulverization that uses an inert gas, such as nitrogen (e.g. Japanese Patent No. 1883860). This technique produces a powder whose grain-size distribution has a sharp peak.
[Molding]
The technique of creating a sintered magnet having a magnetic anisotropy by compression molding of a powder in a magnetic field was initially adopted in the invention of a ferrite magnet (Examined Japanese Patent Publication No. S29-885 or U.S. Pat. No. 2,762,778) and later applied to the production of RCo or RFeB magnets (U.S. Pat. No. 3,684,593 or Japanese Patent No. 1431617). The fine particles of the powder are compacted into a body in which their c-axes of the RFeB tetragonal crystal structure are oriented to the same direction. A typical technique is the die-pressing method. Other methods include the CIP method (Japanese Patent No. 3383448) and the RIP method (Japanese Patent No. 2030923), both of which provide higher degrees of orientation and larger energy products.
[Die-Pressing Method]
In 1951, when Went et al. invented a ferrite magnet (Examined Japanese Patent No. S35-8281 and U.S. Pat. No. 2,762,777), Gorter et al. also invented a sintered ferrite magnet having a magnetic anisotropy (Examined Japanese Patent No. S29-885 and U.S. Pat. No. 2,762,778). This was the first case where the compression molding in a magnetic field was combined with a sintering process to manufacture a permanent magnet having a magnetic anisotropy. Since then, various improvements have been made to overcome the problems discerned in the mould-pressing method.
[Addition of a Lubricant]
In some methods, a lubricant is added to increase the degree of orientation of the fine particles during the die-pressing process and to reduce the friction among and between the particles and the die (e.g. Japanese Patent Nos. 2545603 and 3459477).
[Wet Pressing in a Magnetic Field]
To achieve a high degree of orientation while preventing the fine particles from oxidization, some methods include the steps of mixing the fine particles with a mineral oil, a synthetic oil or a vegetable oil, injecting the mixture into the die with a high pressure, and performing a wet compression molding in a magnetic field (e.g. Japanese Patent No. 2731337). Some reports on this technique claim that the magnetic characteristics can be improved by pressure injection and pressure compression of slurry (Japanese Patent No. 2859517).
[CIP]
The die-pressing method can apply the pressure only in one direction, which leads to misorientation. An isotropic application of pressure from every direction will reduce the disorder of the orientation. In one method for the isotropic application of the pressure, a rubber container filled with the fine particles is set in an external magnetic field and subjected to a cold isostatic pressing (CIP) process (Japanese Patent No. 3383448).
[RIP]
To obtain the same effect as the CIP method, the present inventors proposed the rubber isostatic pressing (RIP) method, in which a rubber mold is set in a die-pressing machine and subjected to an isostatic pressure (Japanese Patent No. 2030923). This method is easier to automate and hence far more suitable for mass production than the CIP method.
[AT]
An air-tapping [AT] method, which was proposed in Unexamined Japanese Patent Publication Nos. H09-78103, H09-169301 and H11-49101, is a method for loading a cohesive fine powder into the die cavity of a die-pressing machine or similar machines. In this technique, a rapid flow of air is intermittently supplied onto a powder to uniformly load it into the die cavity with high density. In a method proposed in Unexamined Japanese Patent No. 2000-96104, the air-tapping technique is used to solidify the powder into an object having a near net shape.
[Pulsed Magnetic Field]
A magnetic field is externally applied to the particles in order to orient them in the same direction. In the case of RFeB magnets, the c-axis of the tetragonal crystal structure corresponds to the easy magnetization axis. When the magnetic field is applied, the particles are oriented in the axial direction. Normal types of die-pressing machines use an electromagnet to create a static magnetic field, whose maximum field strength is about 15 kOe. In contrast, in the case of creating a pulsed magnetic field with an air core coil, the field strength can be as high as 15 to 55 kOe. Use of such a strong magnetic field actually improves the magnetic characteristics (Japanese Patent No. 3307418).
[Closed System]
In a method proposed in Unexamined Japanese Patent Publication No. H06-108104, the pulverization and molding processes are performed under inert atmosphere in order to avoid the powder oxidization.
[Patent Document 1] Japanese Patent No. 1431617